1. Field of the Invention
The present invention relates to a balun transformer used for a balance-unbalance signal converter for converting balanced signals into an unbalanced signal and vice versa, a phase converter and the like, and more particularly to a compact, broadband balun transformer and a mounting structure for a balun transformer and an electronic apparatus having a built-in mounting structure.
2. Description of Related Art
A balun transformer is used, for example, as a converter for converting balanced signals into an unbalanced signal or vice versa. Balanced lines have a pair of signal lines, and a balanced signal is propagated as a potential difference between paired signal lines. An unbalanced line is used for an unbalanced signal propagated as a potential at one signal line relative to a ground potential. As an unbalanced signal is input to an unbalanced terminal of a balun transformer, two balanced signals having a phase difference (opposite phases) of 180° and the same amplitude are output from balanced terminals of the balun transformer. A mobile communication apparatus such as a portable phone uses the balun transformer as a balance-unbalance signal converter. The balun transformer is also called a balun and a balun circuit.
There are many reports on a balun transformer.
The following description is given in Patent Document 1: Japanese Patent Application Publication No. 2000-58328 (paragraphs 0010 and 0011) titled “Stacked Type Balun Transformer”.
The stacked type balun transformer regarding the invention described in Patent Document 1 includes at least a pair of first and second strip lines electromagnetically coupled via a dielectric layer and facing each other in a magnetically coupled area and a pair of third and fourth strip lines electromagnetically coupled via a dielectric layer and facing each other in an magnetically coupled area. The two pairs of strip lines are stacked with a dielectric layer in between, and a ground electrode facing the strip line is stacked having a dielectric layer in between at least at one of a position between the two pairs of strip lines, a position above the two pairs of strip lines, and a position under the two pairs of strip lines. One end of the first strip line and one end of the fourth strip line are electrically connected; the other end of the first strip line is electrically connected to an input/output external electrode; the other end of the fourth strip line is opened; one end of the second strip line is electrically connected to the input/output external electrode and the other end thereof is electrically connected to the ground electrode; and one end of the third strip line is electrically connected to the input/output external electrode and the other end thereof is electrically connected to the ground electrode.
With this structure, each strip line is not juxtaposed on the same dielectric layer, but each strip line is stacked with a dielectric layer positioned in between so that a balun transformer of a small area can be realized. Further, a thickness of the dielectric layer sandwiched between the pair of strip lines electromagnetically coupled can be adjusted independently from the dielectric layer sandwiched between the other pair of strip lines.
The following description is given in Patent Document 2: Japanese Patent Application Publication No. 2003-7538 (paragraphs 0019 and 0020, paragraphs 0024 and 0025, FIG. 3) titled “Stacked Type Balun Transformer”.
FIG. 15 of the present specification corresponds to FIG. 3 of Patent Document 2, and is an electrical equivalent circuit of the first embodiment of the stacked type balun transformer of Patent Document 2.
A balanced signal terminal 242a is electrically connected to one end of a first line 225, a balanced signal terminal 242b is connected to one end of a second line 228, and a relay terminal 243 is electrically connected to ends of a third line 224 and a fourth line 227.
In the balun transformer 221, the third and first lines 224 and 225 are disposed between the ground electrodes to constitute a strip line structure. The fourth and second lines 227 and 228 are also disposed between the ground electrodes to constitute a strip line structure. The third and fourth lines 224 and 227 are serially connected via the relay terminal to constitute an unbalanced transmission line 238. The first and second lines 225 and 228 constitute balanced transmission lines 239 and 239. The lines 224 and 225 and the lines 227 and 228 are formed facing each other with a dielectric sheet being interposed therebetween.
Although one end of the unbalanced transmission line 238 (specifically, at the end of the line 227) is opened, this end may be set as a ground end. The balun transformer 221 has upper and lower ground electrodes so that the shielding effect is provided.
Next, with reference to FIG. 15, description will be made on a case in which the balun transformer 221 is used as a balance-unbalance signal converter. As an unbalanced signal S1 is input to the unbalanced signal terminal 241, the unbalanced signal S1 propagates the unbalanced transmission line 238 (line 224-relay terminal 243-line 227). The line 224 line-couples the line 225, and the line 227 line-couples the line 228. Therefore, the unbalanced signal S1 is converted into balanced signals S2 which are output from the balanced signal terminals 242a and 242b. Conversely, as the balances signals S2 are input to the balanced signal terminals 242a and 242b, the balanced signals S2 propagates the balanced transmission lines 239 and 239 and converted into the unbalanced signal S1 at the unbalanced transmission line 238 which is then output from the unbalanced signal terminal 241.
The following description is given in Patent Document 3: Japanese Patent Application Publication No. 2003-8312 (paragraphs 0003 to 0006, paragraphs 0013 to 0015, FIGS. 1 to 3) titled “Balun Transformer”.
FIGS. 16A to 16C of the present specification correspond to FIGS. 1 to 3 of Patent Document 3. FIG. 16A is a circuit diagram of a balun transformer according to an embodiment of the invention of Patent Document 3, and FIGS. 16B and 16C are circuit diagrams showing examples of an existing balun transformer.
Some of balun transformers interconnecting an unbalanced transmission line and balanced transmission lines to transmit a high frequency signal use distributed constant transmission lines. Examples of an existing balun transformer of this type are shown in the circuit diagrams of FIGS. 16B and 16C.
In FIGS. 16B and 16C, a first transmission line 310 has a length of about a quarter wavelength or shorter of a high frequency signal. A second transmission line 311 having a length of about a quarter wavelength or shorter is disposed parallel to the first transmission line 310 on the same plane or three-dimensionally parallel to the first transmission line and electromagnetically coupled to the first transmission line. An input terminal IN is an unbalanced terminal provided to the first transmission line 310, and output terminals OUT1 and OUT2 are balanced terminals provided to opposite ends of the second transmission line 311. In the example shown in FIG. 16B, a center of the second transmission line 311 is grounded, and in the example shown in FIG. 16C, the second transmission line 311 is not grounded.
As shown in FIGS. 16B and 16C, a balun transformer constituted of existing transmission lines uses the first transmission line 310 whose one end is used as unbalanced input terminal IN and whose other end is grounded, and the second transmission line 311 whose opposite ends are used as the balanced output terminals OUT1 and OUT2, and has either the structure that the center of the second transmission line 311 is grounded as shown in FIG. 16B or the structure that the second transmission line 311 is not grounded as shown in FIG. 16C.
The existing balun transformer of this type is, however, associated with the following problem. If an unbalanced signal is input to the unbalanced signal input terminal IN of the balun transformer, in the examples shown in FIGS. 16B and 16C, the positional relation between the input terminal IN and output terminal OUT1 are in a proximity positional relation so that the input terminal IN and output terminal OUT1 are capacitively coupled. Therefore, a phase shift from the opposite phases and an amplitude level difference between two balanced signals picked up from the output terminals OUT1 and OUT2 of the balun transformer become larger as the frequency becomes higher.
FIG. 16A is a circuit diagram showing an example of the balun transformer of the embodiment of Patent Document 3. In FIG. 16A, a first transmission line 301 has a length of about a quarter wavelength or shorter of a high frequency signal. A second transmission line 302 and a third transmission line 303 having a length of about a ⅛ wavelength or shorter are disposed parallel to the first transmission line 301 on the same plane or three-dimensionally parallel to and straight to the first transmission line and electromagnetically coupled to the first transmission line. An unbalanced terminal IN as an input terminal of an unbalanced signal is provided at one end of the first transmission line 301. Balanced terminals OUT1 and OUT2 for balanced signals are provided to confronting ends of the second and third transmission lines 302 and 303. The first transmission line 301 has the unbalanced terminal IN at its one end, and the other end is grounded. The second and third balanced lines 302 and 303 have the balanced terminals OUT1 and OUT2 at the confronting ends thereof, and the other ends are grounded. The first to third transmission lines 301 to 303 are formed on the surface of or in a dielectric substrate which is not shown in FIG. 16A.
The balun transformer of Patent Document 3 having the structure described above uses as the dielectric substrate including a multi-layer dielectric substrate formed by stacking a plurality of dielectric layers. The first to third transmission lines 301 to 303 are made of microstrip lines, strip lines or coplanar lines formed on the surface of or in a multi-layer substrate having a plurality of dielectric layers. The other ends of the first to third transmission lines 301 to 303 are grounded to an external ground via through conductors such as through hole conductors and via conductors formed in the dielectric substrate and/or terminal electrodes formed on the side walls of the dielectric substrate and made of metalized conductor layers, so-called casterllation conductors or the like. In this manner, the balun transformer which is compact and has excellent high frequency transmission characteristics can be formed on the surface of or in the dielectric substrate. This balun transformer can be formed integrally with a high frequency circuit and is suitable for the high frequency circuit.
In forming the balun transformer of the invention described in Patent Document 3, the dielectric substrate typically a multi-layer dielectric substrate formed by stacking a plurality of dielectric layers, the first to third transmission lines 301 to 303 made of microstrip lines, strip lines, coplanar lines or the like, the unbalanced terminal IN, the balanced terminals OUT1 and OUT2, through conductors, terminal electrodes can be formed by using various materials and types used in well-known high frequency wiring substrates.
The following description is given in Patent Document 4: Japanese Patent Application Publication No. 2002-33216 (paragraphs 0008 to 0010) titled “Stacked Balun Transformer”.
An object of the invention of Patent Document 4 is to provide a stacked balun transformer having good coupling in a broadband, particularly at a high frequency of 1 GHz or higher.
The stacked balun transformer of Patent Document 4 is a stacked balun transformer having at least two pairs of λ/4 strip lines electromagnetically coupled, and is characterized in that a non-magnetic layer containing polyvinyl benzyl ether compound is disposed between paired strip lines electromagnetically coupled, and that a magnetic layer made of polyvinyl benzyl ether compound dispersed with magnetic powders is disposed above and under a composite layer constituted of the paired strip lines and the non-magnetic layer.
Since the non-magnetic layer containing polyvinyl benzyl ether compound is disposed between paired strip lines, capacitive components between the strip lines can be reduced because the polyvinyl benzyl ether compound has a low relative dielectric constant of about 2.5 to 3.5. Furthermore, the number of crossed fluxes between strip lines can be increased because of non-magnetism so that a coupling coefficient increases. Polyvinyl benzyl ether compound can be manufactured easily by the epoxy resin manufacturing process. There is no problem of cracks and warps as in the case of baking ceramic.
The following description is given in Patent Document 5: Japanese Patent Application Publication No. 2005-306696 (paragraphs 0008 and 0009) titled “Magnetic Ferrite, Common Mode Noise Filter Using Magnetic Ferrite, and Chip Transformer”.
The invention of Patent Document 5 uses magnetic ferrite having the main components of Fe, Co and Zn having compositions in the range of 39.5:53.0:7.5 mol %, 39.5:48.0:12.5 mol %, 20.0:67.5:12.5 mol %, and 20.0:55.0:25.0 added with copper oxide of 8 to 14 wt %.
The magnetic ferrite, a common mode noise filter and a chip transformer using the magnetic ferrite can realize magnetic ferrite which has a low loss in a high frequency band and can be baked together with silver electrodes, and can realize a high frequency common mode noise filter and chip transformer having a large coupling coefficient between two coils, by using the magnetic ferrite in stacked electronic components.